With the alternatives for sludge processing changing because of the public awareness of the problems of sludge dumping, either in landfills or oceans, the treatment of municipal sludges by a sterilization or a pasteurization process is becoming increasingly common so that it is safe for exposure to the public as a product. Unfortunately many of these treated sludges do not possess a long term stability against the formation of noxious odors and or the regrowth of a pathogenic microflora which would pose a potential and significant public health threat. Me present invention introduces a long term stability to such treated wastewater sludges as well as untreated wastewater sludges, e.g., raw primary or secondary sludges.
The United States Environmental Protection Agency (EPA) has promulgated rules governing the type of processes that an be used to treat wastewater sludge.
Under 40 CFR 257, a Process to Further Reduce Pathogens (PFRP) must be used where sewage sludge or septic tank pumping are to be applied to a land surface or are incorporated into the soil, and crops for direct human consumption are to be grown on such land within eighteen (18) months subsequent to application or incorporation.
A Process to Significantly Reduce Pathogens (PSRP) must be used where sewage sludge or septic tank pumping are to be applied to a land surface or incorporated into the soil and the public will have access to such land within twelve (12) months subsequent to application or incorporation, or grazing animals, whose products are consumed by humans, will have access to such land within one (1) month subsequent to application or incorporation.
40 CFR 257 classifies the following as PSRP and PFRP processes:
A. Processes to Significantly Reduce Pathogens
Aerobic digestion: The process is conducted by agitating sludge with air or oxygen to maintain aerobic conditions at residence times ranging from 60 days at 15.degree. C. to 40 days at 20.degree. C., with a volatile solids reduction of at least 38 percent.
Air Drying: Liquid sludge is allowed to drain and/or dry on under-drained sand beds, or paved or unpaved basins in which the sludge is at a depth of nine inches. A minimum of three months is needed, two months of which temperatures average on a daily basis above 0.degree. C.
Anaerobic digestion: The process is conducted in the absence of air at residence times ranging from 60 days at 20.degree. C. to 15 days at 35.degree. C. to 55.degree. C., with a volatile solids reduction of at least 38 percent.
Composting: Using the within-vessel, static aerated pile or windrow composting methods, the solid waste is maintained at minimum operating conditions of 40.degree. C. for 5 days. For four hours during this period the temperature exceeds 55.degree. C.
Lime Stabilization: Sufficient lime is added to produce a pH of 12 after 2 hours of contact.
Other methods: Other methods of operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.
B. Process to Further Reduce Pathogens
Composting: Using the within-vessel composting method, the solid waste is maintained at operating conditions of 55.degree. C. or greater for three days. Using the static aerated pile composting method, the solid waste is maintained at operating conditions of 55.degree. C. or greater for three days. Using the windrow composting method, the solid waste attains a temperature of 55.degree. C. or greater for at least 15 days during the composting period. Also, during the high temperature period, there will be a minimum of five fumings of the windrow.
Heating drying: Dewatered sludge cake is dried by direct or indirect contact with hot gases, and moisture content is reduced to 10 percent or lower. Sludge particles reach temperatures will in excess of 80.degree. C. or wet bulb temperature of the gas stream in contact with the sludge at the point where it leaves the dryer is in excess of 80.degree. C.
Heat treatment: Liquid sludge is heated to temperatures of 180.degree. C. for 30 minutes.
Thermophilic Aerobic Digestion: Liquid sludge is agitated with air or oxygen to maintain aerobic conditions at residence times of 10 days at 55.degree.-60.degree. C., with a volatile solids reduction of at least 38 percent.
Other methods: Other methods of operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.
Any of the processes listed below, if added to the processes described in Section A above, further reduce pathogens. Because the processes listed below, on their own, do not reduce the attraction of disease vectors, they are only add-on in nature.
Beta ray irradiation: Sludge is irradiated with beta rays from an accelerator at dosages of at least 1.0 megarad at room temperature (ca. 20.degree. C.).
Gamma ray irradiations: Sludge is irradiated with gamma rays from certain isotopes, such as .sup.60 Cobolt and .sup.137 Cesium, at dosages of at least 1.0 megarad at room temperature (ca. 20.degree. C.).
Pasteurization: Sludge is maintained for at least 30 minutes at a minimum temperature of 70.degree. C.
Other methods: Other methods of operating conditions may be acceptable if pathogens are reduced to an extent equivalent to the reduction achieved by any of the above add-on methods.
In U.S. Pat. Nos. 4,781,842 and 4,902,431 there is disclosed processes wherein:
In these processes, the alkaline material may comprise lime, cement kiln dust or lime kiln dust or other alkaline materials.
Other processes for treating wastewater sludge have utilized the concept of raising the pH in combination with high heat, e.g., gearer than 70.degree. C., to sterilize as contrasted to pasteurizing the sludge thereby killing all of the bacteria both undesirable and desirable. With these "add-on" processes usually the principal surviving microorganisms are bacterial spores. Such microbially restricted sludges lose the significant fertility value associated with bioactivity. When alkaline materials are added to a sludge to raise the pH, a toxicity may exist due to the high pH. When the product is used as a soil supplement in agriculture, particularly at high application rates, there is a risk of over alkalinization of the soil (see FIG. 14) and burning of crops may result. In addition, a high pH (over pH 1 1) in the soil due to the addition of active alkaline materials containing calcium oxide or metal hydroxides can result in severe damage to microbial populations in surface soils. With most existing (traditional) alkaline technologies it has been required by the USEPA that the pH be maintained above pH 12 to prevent microbial overgrowth and instability. In fact, with the PFRP "add-on" heat processes, the pH is required by the USEPA to be maintained above 12 until the alkaline treated sludge is land applied. This requirement is based upon the recognition that when such sludges fall below pH 11 noxious odors will develop. U.S. Pat. No. 4,902,431 column 2 line 58-67 states: "In January 1979, the EPA published a Wastewater Sludge Manual (EPA 625/1-79-001) tided `Process Design Manual for Sludge Treatment and Disposal` which states: `Lime stabilizations a very simple process. Its principal advantages over other stabilization processes are low cost and simplicity of operation . . . lime addition does not make sludges chemically stable; if pH drops below 11.0, biological decomposition will resume producing noxious odors.`"
In addition, the high pH triggers the release of volatile ammonia nitrogen from the sludge which also is toxic and results in the loss of valuable nitrogen from the potential agricultural product. Further, the toxic nature of ammonia, i.e., to human and animal mucus membranes has been described as well as its lethal activity on microorganisms (see Meehan et al 1988 U.S. Pat. No. 4,793,927). Although having ammonia present during sludge stabilization processing is highly desirable for microbial control, it is not desirable following treatment when the sludge product usage and exposure to the public is likely.
If these toxic stresses and the residual odor in a sludge product could be reduced upon demand, then opportunities for utilization of alkaline sludge products by the public and private sector would increase. This result would be favorable to increased emphasis on resource recovery of the value inherent in municipal sludge material.
The USEPA has evaluated lime stabilization (EPA 600/2-78-171; EPA 670/2-75-012; and Farrell et al., 1974) and has established the cost effectiveness, simplicity, and high level of performance of lime stabilization particularly PFRP alkaline processes for wastewater sludges. The process of U.S. Pat. No. 4,781,842; 4,902,431 requires a drying period which is usually effected by a windrowing process and results in a product that is above pH 12 and, if produced from an anaerobically digested sludge, emits significant amounts of ammonia. However the processes substantially reduce the emission of ammonia by aeration (such as windrowing) but to do so the processes are taking 3 to 10 days to prepare the product for storage or market. As evidence of health concerns over ammonia, states such as Ohio, New Jersey and California have implemented air quality standards regulating the emission of ammonia from industrial sites. Recently other alkaline treatment processes such as the RDP Company's Envessel Process (trademark) - (U.S. Pat. No. 5,013,458) using high amounts of CaO and heat above 70.degree. C. for 30 minutes have been approved by the USEPA as PFRP, however these sludge products made by these processes have been required by the USEPA in 1990 to be maintained at a pH of 12 or above for stability and inhibition of odor development. This USEPA policy prevents these alkaline treated sludges from developing a stabilizing and enriching microflora, causes them to emit excess ammonia and requires the sludge to be maintained in a high alkaline condition with substantial toxic hydroxide present that is detrimental to direct application to soils and crops. It is also significant with regard to the proposed invention that traditional alkaline stabilization is still being used as PSRP treated sludges may still be land applied. Other more refined PSRP treatment processes such as the Chemfix (trademark) of Chemfix Technologies, Incorporated Process (U.S. Pat. No. 3,837,872) are also being used in various municipalities. Both of these processes could be modified to a sterilization or pasteurization process by the inclusion of additional calcium oxide [U.S. Pat. Nos. 4,270,279 (sterilization) and U.S. Pat. No. 4,781,842; 4,902,431 (pasteurization)]; and then subsequently stabilized as taught by the present invention or they can be stabilized using the present invention without achieving PFRP levels of disinfection.
It is fact that when alkaline materials are added to municipal sludges in sufficient mass to raise the pH to at least 11 and often to over 12 that several toxic stresses occur that may affect the disposition of the treated sludge material (EPA 600/2-78-17 1). For example, the high pH itself may preclude the product use in certain agricultural settings; in addition, the high pH triggers the release of volatile ammonia which itself is toxic and, of course, represents the loss of valuable nitrogen from the potential agricultural product. The toxic nature of ammonia, i.e., to human and animal mucus membranes has been described as well as its lethal activity on microorganisms (see Meehan et al. 1988, U.S. Pat. No. 4,793,927). Although having ammonia present during sludge stabilization processing is highly desirable, it is not desirable following treatment when sludge product usage and exposure to the public is likely. If these toxic stresses could be reduced upon demand then opportunities for alkaline sludge utilization by the public and private sector would increase. This result would be favorable to increased emphasis on resource recovery of the value inherent in municipal sludge material. The present invention not only significantly reduces the ammonia emissions from treated product it also allows for any ammonia product during processing to be collected and scrubbed from the air (see FIG. 9) resulting in an environmentally clean operation.
Time is also important in processing and in determining the use of sludges in various markets. The invention described herein would allow containment of the total process operation thereby facilitating emissions control as discussed above and would shorten the time necessary to produce sludges which will maintain stability under any climatic condition.
In U.S. Pat. No. 4,781,842 and 4,902,431 Nicholson and Burnham teach the significant advantages of adding accelerated drying by aeration to alkaline treated sludges to achieve odor reduction and control. When windrows are used, this Nicholson and Burnham process commonly takes between 3 and 10 days to effect the aeration/drying. The present invention provides a faster method of accomplishing same.
Sontheimer (U.S. Pat. No. 3,345,288) in 1967 showed that CO.sub.2 could be used to neutralize a watery sludge (1.5 to 4.5% solids) so that it could be dewatered more efficiently. He does not teach the neutralization of sludge cakes for the purpose of increased stability. Further, Roediger (U.S. Pat. No. 4,270,279) points out that "the CO.sub.2 in the air may also react with the lime so that the outer surfaces of the particles in the bulk matter of the sludge become rigidified". He also failed to recognize or teach the conditioning step to preparing the sludge for the development of a microbial population that is responsible for long term stability. Wurtz (U.S. Pat. No. 4,997,572) described a very similar process wherein the granulating ability of gaseous CO.sub.2 was important in their sludge pelletization process.
These three patents: 1) fail to recognize the detoxifying abilities of carbonated treatment of alkaline sludges; and most importantly, 2) do not recognize that reduction of the pH of the treated sludges will have a major beneficiating influence on the ability of seeded or indigenous microflora to establish an ecologically active population. This microflora is critically significant to long-term sludge stability because of its ability: a) to enhance by its own metabolism the carbonation of any residual hydroxides or likewise the catabolism of unstable organics; b) to reduce sludge odors and produce a soil-like odor; and c) to prevent the regowth of pathogenic microorganisms.
Among the objectives of the present inventions are to provide a method of treating and stabilizing wastewater sludges rapidly to provide a beneficiated soil or fertilizer; which can be utilized to heat untreated sludge or previously treated sludges, i.e., PSRP or PFRP sludges.